Embodiments of the present disclosure relate to a downhole tool for measuring the angular position of rotatable equipment located in a subterranean borehole
Measurement while drilling (MWD) and logging while drilling (LWD) tools are commonly used in oilfield drilling applications to measure physical properties of a subterranean borehole and the geological formations through which it penetrates. Such MWD/LWD techniques include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, acoustic calliper, downhole pressure, and the like. Formations having recoverable hydrocarbons typically include certain well-known physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range.
In some drilling applications it is desirable to determine the azimuthal variation of particular formation and/or borehole properties (i.e. the extent to which such properties vary about the circumference of the borehole). Such information may be utilized, for example, to locate faults and dips that may occur in the various layers that make up the strata. In geo-steering applications, such “imaging” measurements are utilized to make steering decisions for subsequent drilling of the borehole. In order to make correct steering decisions, information about the strata is generally required. As described above, such information may possibly be obtained from azimuthally sensitive measurements of the formation properties. Azimuthal imaging measurements typically make use of the rotation of the drill string (and therefore the LWD sensors) in the borehole during drilling.
In the present context, by azimuthal position we mean angular position, at a measurement tool in borehole, around the longitudinal direction of the borehole relative to the direction of the Earth's magnetic field. More particularly, the azimuthal reference plane is a plane centred at the measurement tool and perpendicular to the longitudinal direction of the borehole at that point. This plane is fixed by the particular orientation of the measurement tool at the time the relevant measurements are taken. An azimuth is the angular separation in the azimuthal reference plane from a reference point to the measurement point. The azimuth is typically measured in the clockwise direction, and the reference point can be magnetic north.
Magnetometers are conventionally been used to measure azimuthal position in boreholes. However, magnetometers can suffer from poor signal-to-noise ratios, particularly when measuring angular positions affected by relatively high frequency variations in rotation speed.
For example, FIG. 1 shows a frequency spectrum derived from magnetometer measurements of variation in rotational speed at a subterranean location along a particular drillstring. At frequencies of less than about 8 Hz, the measurements from the magnetometers reflect well the actual dynamics of the drillstring. However, at frequencies above about 8 Hz, the magnetometers merely measure noise, and the actual dynamics of the drillstring are lost.
Therefore, there exists a need for an improved tool for measuring angular positions, e.g. for correlation with directional formation evaluation measurements.